Academician Wu Feng's team Nano Energy: Cascaded sodium storage two-dimensional SeSI cathode: both high specific capacity and long cycle performance
【Original Title】
Cascaded sodium storage two-dimensional SeSI cathode
First published: June 2, 2024
Authors: Qian Mengmeng, Wu Feng, Yu Chuguang, Zhang Junfan, Wang Tong, Wang Jing, Song Tinglu, Tan Guoqiang
Unit: School of Materials, Beijing Institute of Technology, Chongqing Innovation Center, Beijing Institute of Technology
【Background】
Given the continuous consumption of non-renewable energy sources, there is an increasing demand for sustainable and affordable energy storage solutions. As an emerging energy storage system, sodium-ion batteries (SIBs) are regarded as a potential alternative to lithium-ion batteries due to their abundant sodium resources and low cost. However, SIBs face important challenges, such as the relatively large ionic radius of sodium ions during electrochemical cycling, which leads to obvious volume changes and slow redox kinetics of the electrodes, which seriously affects the cycling performance and rate performance of SIBs, and limits their practical application. Although researchers have improved the electrochemical performance of sodium storage through nanostructure optimization and carbon modification, it is still challenging to find new SIBs electrode materials with excellent cycling performance and high specific capacity. Therefore, the development of new cathode materials with enhanced properties is essential to advance the development of SIBs technology.
【Introduction】
Although sodium polysulfide is less soluble in carbonate solvents than lithium polysulfide, it helps to reduce the occurrence of side reactions; The electronic conductivity and ionic conductivity of sodium polyselenide are higher than those of lithium polyselenide, which can improve the reaction kinetics of sodium-ion batteries, however, the energy output and cycle life of single-element Na–S, Na–Se and Na–I2 batteries are poor. Based on the above situation, the team of Academician Wu Feng and Professor Tan Guoqiang of Beijing Institute of Technology recently published a research work entitled "Two-dimensional medium-entropy SeSI composite cathodes with cascaded redox reactions for advanced sodium batteries" in the internationally renowned journal Nano Energy. This work proposes a novel ternary conversion Na−SeSI@C cascade battery configuration, which combines three successive redox reactions in a single cell system, aiming to achieve high capacity and high voltage, thereby achieving high specific energy output in SIB.
The first author of the paper is Qian Mengmeng, a doctoral student at Beijing Institute of Technology, and Academician Wu Feng and Professor Tan Guoqiang are the co-corresponding authors of this paper.
Figure 1. Schematic diagram of the synthesis process of the SeSI@C and (b) SeSI@C-carbon cloth electrodes.
Figure 1 illustrates the synthesis process of an electrode. Se and I2 were heated under a CS2 atmosphere to rapidly melt and convert to SeI, while the gaseous CS2 was reduced to carbon and tightly coated on the SeSI complex to form a two-dimensional SeSI@C-core-shell nanostructure. In order to achieve higher electrode conductivity to meet the diverse market demand, we add carbon cloth as the reaction substrate in the above reaction process to construct a SeSI@C-carbon cloth composite structure in situ, which is expected to realize the practical application of SeSI@C electrode materials in special scenarios.
[Abstract analysis]
Point 1 A one-step chemical substitution strategy was used to construct multi-element conversion composite cathode materials containing Se, S, I and C simultaneously. The core-shell nanostructure can improve the conductivity of the SeSI@C composite electrode and inhibit the dissolution of active species in the electrolyte, which helps to achieve its excellent electrochemical performance.
Figure 2. Morphology and structural characterization of Se2SI1.5@C. (a) TEM plots of typical Se2SI1.5@C nanosheets, corresponding (b) SAED and (c) TEM plots showing Se2SI1.5@C core-shell structures, (d) HRTEM (e) STEM and EDS mapping plots (f) XRD plots of pure S, Se, I2 and Se2SI1.5@C (g) Raman spectra of Se2SI1.5@C (h-k) Se2SI1.5XPS fine spectra and fitting curves for four elements in @C.
Figure 2 illustrates the crystal structure and chemical composition of the Se2SI1.5@C. The high-power transmission image shows the core-shell structure of the Se2SI1.5@C. The core is a crystalline Se2SI1.5@C and the outside is an amorphous carbon structure with a thickness of 2-3 nm. Raman, XPS data prove that Se2SI1.5@C is not a simple physical mixture of four substances, S, Se, I2 and C, but a complex with covalent bonds between the elements.
Point 2 The presence of Se2SI1.5@C covalent bonds enhances the stability of the structure, and the mutual bonding between elements promotes rapid ion and electron transport, which is conducive to the excellent electrochemical performance of the composite cathode.
Fig. 3.(a-c) Morphology and structure characterization of three Se2SIx@C-carbon cloths with different iodine contents. (d) AFM and height plots of Se2SI@C (e) SEM plots of Se2SI@C-carbon cloth (f) Partial enlarged plots of panel e; (g-h) EDS mapping of Se2SI@C (a) TEM map of Se2SI@C; (b) A typical Se2SI@C STEM image, (c) SAED and (d) HRTEM plot showing the core-shell structure of the Se2SI@C and corresponding (e) STEM and EDS mapping plots.
In order to achieve rapid electron and ion transport while increasing the loading of active species, we synthesized SeSI@C-carbon cloth self-supporting electrodes in situ on a three-dimensional (3D) carbon cloth made of carbon fiber (Figure 3). This self-supporting electrode design without the need for additional binders and conductive agents is considered an effective way to achieve high active mass loading. Among them, the carbon cloth is composed of a 3D interconnected carbon fiber structure, and its unique three-dimensional conductive network provides a large specific surface area, which is conducive to the wetting of the electrolyte. The voids of carbon fibers can be used to accommodate the volumetric expansion of the SeSI@C as well as the accumulation and adsorption of the intermediates NaPSes/NaPSs/NaPIs. The SeSI@C grown on the carbon fibers exhibits a ribbon-like nanosheet structure. With a highly reactive surface and low contact resistance, these ultra-thin SeSI@C nanoribbons provide high specific surface area and fast electron transport.
Point 3 By constructing the SeSI@C-carbon cloth composite electrode material in situ on the carbon cloth, a coaxial hierarchical structure is formed, which is conducive to achieving low resistance and efficient ion and electron transfer paths, and further improving the energy density and cycling stability of the electrode material.
Figure 4 (a-c) CV curves (first four weeks) and (d-f) CV curves (g-i) of three Na-Se2SIx@C-carbon cloth cells with different iodine contents, GITT curves of the cells, and diffusion coefficients of Na+ ions.
The electrochemical performance of Se2SIx@C was evaluated in the battery using cyclic voltammetry (CV) and constant current batch titration (GITT). Se2SI@C, Se2SI1.5@C, and Se2SI2@C have the same electrochemical behavior (Figure 4). The conversion reaction of Se2SIx@C can be expressed as: SeSI⇆NaI3+Na2Sn+Na2Sen⇆NaI+Na2SeS+Na2S. CV curves obtained with sweep velocities ranging from 1.0 to 4.0 mV s-1 show that the Se2SI2@C with the highest I2 content has almost no significant peak shift, which may be due to the excellent redox reversibility of I2/I3-. At the same time, the Na+ ion diffusion coefficients in the three electrodes were calculated based on the GITT curve. The Se2SI@C, Se2SI1.5@C, and Se2SI2@C electrodes all exhibit Na+ ion diffusion coefficients of up to 10-10 cm2 s-1, confirming the rapid charge transfer kinetics.
Figure 5. Electrochemical performance of Na-Se2SI@C-carbon cloth battery (a) Charge-discharge curve at different current densities (b) Rate performance (c) Cycling performance under different loads (d) Long cycle test at 10.0 A g-1 (e) Performance comparison chart (f) ToF-SIMs diagram of Se2SI@C-carbon cloth electrode before and after reaction (h) Se2SI@C-carbon in the fully charged state Three-dimensional view of the cloth's ToF-SIMs: (i) cycled TEM images, (j) HRTEM and SAED plots, and (k) STEM images of individual Se2SI@C nanosheets with corresponding (i) element distribution maps.
The voltage curves of Na−Se2SI@C cells cycling at different current densities are shown in Figure 5. Even at a current density of 10.0 A g-1, it still provides a satisfactory specific capacity of 423.9 mAh g-1, which is almost double the capacity of the nickel-rich cathode in current lithium-ion battery materials. Thanks to its high Na+ diffusion coefficient, the charge-discharge curve maintains a stable voltage plateau even at ultra-high current densities, showing excellent high-rate performance at current densities of 1.0, 3.0, 5.0, 7.0 and 10.0 A g-1, Na-Se2SI@C exhibit excellent specific capacities of 705.5, 615.0, 545.4, 475.6 and 425.6 mAh g-1, respectively. We evaluated the battery performance of Na-Se2SI@C at different active mass loadings (8.0, 9.6, 11.6, 13.5, and 15.6 mg cm-2).
In general, a thin electrode with a lower loaded mass is more likely to obtain a higher specific capacity than a thick electrode with a higher loaded mass during the actual electrochemical performance test due to the more complete reaction of the active species, while a thick electrode may provide a higher area capacity than a thin electrode. For the Se2SI@C-carbon cloth electrode we designed, both thin and thick electrodes exhibit excellent electrochemical performance. Even at high current densities (10 A g-1), after 2000 cycles, the capacitance retention rate is 91.6% and the capacitance attenuation rate is only 0.004%. In addition, ToF-SIMS and TEM were used to characterize the Se2SI@C-carbon cloth electrode after a long period of cycling. After 2000 cycles Se2SI@C still maintains an intact 2D nanosheet electrode structure with a uniform distribution of active species, which further confirms the correlation between cycling performance and structural integrity.
【Conclusion】
The SeSI@C-carbon cloth composite electrode material with high load (8 mg cm−2) and high conductivity was obtained by coupling three successive redox reactions of S, Se and I2 on a carbon cloth in a battery system by one-step chemical displacement method. Thanks to the atomic-level homogeneous mixing of SeSI, the electronic structure and electrochemical activity of Se, S and I2 are effectively adjusted, enabling it to achieve high energy density output in successive multiple electron redox reactions. The SeSI@C cathode has faster reaction kinetics, higher capacitance and discharge voltage than S, Se and I2, and has a better ability to suppress the shuttle effect, showing better electrochemical performance.
The combination of high energy output and long cycle life of ternary SeSI is attributed to: (1) the synergistic effect of the three elements, especially the addition of I2, greatly improves the electronic conductivity and electrochemical activity of the composites, which is conducive to the high-rate performance of sodium batteries. There is no need for external integration of non-electrochemically active connection components, and the multi-synergy effect further improves the energy density and cycling stability of the electrode material. (2) The SeSI@C with single-phase 2D structure has excellent structural stability and electrochemical stability. (3) The incorporation of I2 and the formation of NaI greatly promoted the subsequent reduction of polysulfide and polyselenide, and inhibited the dissolution of active substances, indicating that it played the role of a stabilizer in the electrochemical reaction and was conducive to the cycling stability of Na batteries. (4) The coaxial hierarchical structure of the SeSI@C-carbon cloth can achieve efficient ion and electron transfer paths, and the charge transfer resistance is low.
Therefore, the efficient synergistic effect between S, Se and I elements was used to construct SeSI composites with high conductivity and high specific capacity. The multivariate synergistic effect further improves the energy density and cycling stability of the electrode material.
【Article Link】
Two-dimensional medium-entropy SeSI composite cathodes with cascaded
redox reactions for advanced sodium batteries
https://doi.org/10.1016/j.nanoen.2024.109841